Can someone explain what is/what the advantages to ZFS are?
For a single computer with a single hard drive, there aren't a lot of advantages.
It shines when you are running a large file server. You can gang together lots of hard drives and ZFS will make them appear as one large volume, hopefully without all the ugly management you have to do to set up a RAID file system. You can also add more disks to the existing file system without blowing it away (with RAID, you generally have to do a backup/destroy/create/restore to increase the capacity of an array.)
The other key features mentioned by the Wikipedia article (copy-on-write semantics and snapshots) are useful to any user, but the benefits can be accomplished through other means as well, including the use of journaling and an automatic backup system (like Time Machine.)
I predict that ZFS will become really big for people using XServe RAID and other large-capacity drive arrays. I don't see much point to using it on a computer that has only one hard drive.
Maybe they not been able to get writing to work on Mac OSX. I'll bet that Apple has been testing their ZFS read software using files written by Solaris. I'd also bet Apple has Solaris running on the Mac Pro and on Xserve. Talk about a neat product. They could sell Solaris based storage products today. Apple's prices are very competitive in that market.
That's quite a leap of logic, there.
They may also simply be attaching ZFS volumes formatted on Solaris via SCSI, FireWire or FibreChannel. What makes you think they have to actually run Solaris on the Mac to make this work?